Same deal as with the wide angle zooms like 12-24 / 16-35mm almost invariably being weakest at the long end... sometimes atrociously so.
Maybe the producers are thinking most users have something with overlapping, or at least adjacent zoom range (70-200 in case of the standard zoom, 24-70 in case of the wide zoom). Which is a little unfair (in theory - I don't think I've ever noticed any such weakness in practice) to those who prefer to have a gap, using the 16-35 + 70-200 or 24-70 + 100-400 combos, or even 16-35 + 100-400 like I do most of the time.
Maybe the producers are thinking most users have something with overlapping, or at least adjacent zoom range (70-200 in case of the standard zoom, 24-70 in case of the wide zoom). Which is a little unfair (in theory - I don't think I've ever noticed any such weakness in practice) to those who prefer to have a gap, using the 16-35 + 70-200 or 24-70 + 100-400 combos, or even 16-35 + 100-400 like I do most of the time.